1. Field of the Invention
This invention relates to the processing of periodic information signals, such as, video signals, and more particularly is directed to apparatus by which time base errors introduced during recording and/or reproducing of such signals may be removed.
2. Description of the Prior Art
Video signals are frequently recorded on magnetic tape and subsequently reproduced for later broadcasting or viewing purposes. During the reproduction of recorded video signals, time base or frequency errors are usually introduced by reason of expansion or contraction of the record medium during or after recording, variation in the speed of the tape relative to the magnetic head or heads during recording or reproduction, variation between the tape recording speed and the tape reproducing speed, and the like. The presence of such time base errors in the reproduced video signals cause a frequency shift of the latter which can result in many observable undesirable effects, particularly when the reproduced video signals are to be transmitted or broadcast and may be mixed with live broadcast material that do not have such time base errors. The observable undesirable effects resulting from relatively small time base errors are a smeared or jittery picture with erroneous intensity variations and, in the case of color video signals, improper color display. When the time base errors are large, the reproduced picture will fail to lock horizontally or vertically.
In an existing time base corrector for substantially removing time base errors from video signals, for example, as disclosed in U.S. Pat. No. 3,860,952, issued Jan. 14, 1975, the incoming video signals are converted from analog to digital form and temporarily stored in a memory. Time base errors are removed from the video signals by writing the digitized signals in the memory at a clocking rate which varies in a manner generally proportional to the time base errors, and by fetching or reading out these stored signals at a standard clocking rate. After such reading out of the digitized video signals, the latter are reconverted to analog form and applied to an output terminal. The memory used in the known time base corrector comprises a plurality of memory units each capable of storing a plurality of horizontal lines of video information. A sequence control unit controls the selection of each memory unit for writing and reading so that the sampled video information is sequentially stored by cyclically enabling the plurality of memory units and serially storing one or more lines of digitized video information in each selected memory unit, and further so that, contemporaneously with the storage of sampled video information in a selected memory unit, and sequence control unit enables the video information stored in a different one of the memory units to be sequentially fetched or read out therefrom, with the enabling of the memory units for the reading out of the information stored therein being also effected in a cyclical manner.
Further, it has been proposed that, in a time base corrector as described above, those line intervals of the incoming video signals in which dropouts occur should be omitted from the output of the time base corrector and replaced by previously stored line intervals of similar video information.
However, problems are encountered in the above described time base correctors, particularly when used for correcting time base errors in NTSC color video signals. More particularly, as is well known, the polarity or phase of the chrominance subcarrier is reversed for successive horizontal or line intervals of NTSC color video signals. Therefore, if the time base corrector is designed, as described above, so as to compensate for dropout by substituting for the line interval or intervals containing dropout, a previously stored line interval or intervals of similar video information, the chrominance subcarrier of each substituted line intervals of video information in the output has to have the same polarity or phase as the chrominance subcarrier of the replaced line interval. Thus, the time base corrector requires a relatively complicated circuit arrangement for detecting the phase or polarity of the chrominance subcarrier and for controlling the phase or polarity of the chrominance subcarrier in the output from the time base corrector so as to ensure that the same is reversed for successive line intervals even when compensating for drop out in the incoming video signals.
Furthermore, in time base correctors, as described above, the time base error of the incoming video signals is usually detected from the burst signals of the latter. Therefore, the frequency of the write clock pulse signal modulated by the time base error, that is, the frequency at which the digitized signals are sampled for writing in the memory, is selected to be a whole multiple of the burst or chrominance subcarrier frequency f.sub.c which, in the case of NTSC color video signals, is about 3.58 MHz. It is also necessary that the write clock pulse frequency be in interleaving relation to the horizontal or line frequency f.sub.h of the incoming video signals which is 15.75 KHz in the case of NTSC color video signals. Such interleaving relation is achieved when the write clock pulse frequency is (2n-1)/2 .times. f.sub.h, in which n is any desired integer. However, since the burst frequency 3.58 MHz of an NTSC color video signal is 15.75 KHz .times. 1/2 .times. 455, the interleaving relation of the write clock pulse frequency to the horizontal or line frequency can be achieved when the write clock pulse frequency is selected to be (2N-1) .times. f.sub.c, in which N is any desired integer. Thus, for example, when the write clock frequency is 10.74 MHz, that is, 3.times.f.sub.c for NTSC signals, the incoming video signals are sampled 682.5 times during each horizontal or line interval thereof. This means that, in alternate horizontal or line intervals of the incoming video signals, 682 and 683 words are respectively written in the memory units which are selectively enabled for the writing operation. The different number of words to be stored in the memory units during successive line intervals of incoming NTSC color video signals may result in complexity of the sequence control unit for controlling the writing and reading operations.